The invention relates to an improvement in a the control system of a d.c. motor.
The rotation speed of a d.c. motor can be controlled either by voltage control or by a mechanical governor. A typical voltage control system is illustrated in FIG. 1, wherein a d.c. motor 1 and a control transistor 2 are connected in series across the terminals of a d.c. source E. A variation in the load of the motor 1 is fed back to the base of the control transistor 2 as a control voltage, to thereby change the voltage V.sub.1 developed across the collector and emitter of the transistor so as to maintain the speed or RPM of the motor 1 at a given value. With this voltage control, the series connection of the transistor 2 with the motor 1 causes a power dissipation of V.sub.1 in the transistor, where i represents the motor current, thus resulting in a lower power efficiency. As a consequence, such control is not conducive to a small instrument which utilizes a dry cell or the like as the d.c. source. The mechanical governor control depends on a mechanical displacement of the governor, resulting from a deviation in the RPM of the motor, to provide a mechanical switching of the connection between the power source and the motor, thus maintaining the RPM of the motor a given value. However, while this improves the power efficiency, the mechanical control tends to generate noises and other adverse influences upon the circuit as a result of the switching operation. In addition, it is not easily possible to alter the RPM or angular velocity.